Wind tunnels are well known as a means of simulating aerodynamic forces experienced by airborne bodies such as aircraft and missiles. In general, wind tunnels comprise a support or `sting`, for a scaled down model of an airborne body that it is wished to study. The sting comprises an elongate member which at one end is tapered and which at the other, non-tapered end, is mounted onto a fixed support. The model is mounted onto the sting by insertion of the tapered end thereof deep into a bore provided within the body of the model. There is also provided, along a portion of the sting operative to be inserted and to engage the model, apparatus for measuring aerodynamic forces experienced thereby.
The measuring apparatus typically comprises a plurality of electrical strain gauges and apparatus for converting into forces electrical potential differences measured across the strain gauges. There is also provided apparatus for generating along the wind tunnel a high speed air flow, typically at supersonic speeds.
When a wind tunnel is in operation there exist two types of loading that are experienced by the model. One type of loading results from steady state operation of the wind tunnel at a substantially constant air flow speed. The other type of loading results from a shock wave generated at the starting and stopping phases of operation. It has been found that the shock wave causes a relatively high pressure difference across the model and so results in relatively high forces and moments thereon which may be as much as ten times the normal forces and moments generated under steady state conditions.
The very high forces and moments generated by the shock waves have been found to cause damage to the high resolution measuring apparatus preferred for use, which is generally of rather delicate construction.
One way of preventing the damage caused to the measuring apparatus by the shock waves is by using `heavy duty` apparatus, although this provides less accurate measurements than the more delicate, high resolution apparatus.
Alternatively, it is known to lower the pressure used in the wind tunnel. Although this prevents damage to the high resolution measuring apparatus, it is wasteful of energy.